Binaural masked thresholds for a speech sound [ba] were estimated under two interaural phase conditions (NoSo and NoS pi) in three age groups (infants, preschool children, and adults). A computer-based head-turn with visual reinforcement procedure was used to test the infants. Preschoolers were tested using a modified play audiometry task, and adults were tested conventionally with the same apparatus as used with the infants. Differences in masked threshold as a function of both age and interaural phase condition, and differences in the binaural masking level difference (BMLD) with age, were found when the masker intensity was the same for each group. However, testing of adults with maskers of reduced intensity to compensate for hearing threshold differences relative to the two younger groups (a possible confounding variable in the estimate of the BMLD) eliminated the difference in BMLD between adults and preschoolers. The difference between infants and adults remained statistically significant, suggesting a possible developmental change in binaural hearing early in life. The relevance of developmental change in binaural hearing with respect to both infant auditory processing and the methods we use to assess it are discussed.